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问题 计算题

如图所示，带电粒子进入匀强磁场，垂直穿过均匀铝板，如果R1=20cm，R2=19cm，求带电粒子能穿过铝板多少次。（设铝板对粒子的阻力恒定，粒子的电量不变）

答案

10次

选择题
在84049700中，从右边数起，第一个4表示（   ）
A．4个千B．4个万C．4个十万
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单项选择题

"I would almost rather see you dead. " Bobert S. Cassatt, a leading banker of Philadelphia, shouted when his twenty-year-old eldest daughter announced that she wanted to become an artist. In the 19th century, playing at drawing or painting on dishes was all right for a young lady, but serious work in art was not. And when the young lady’ s family racked among the best of Philadelphia’s social families, such an idea could not even be considered.

That was how Mary Cassatt, born 1844, began her struggle as an artist. She did not tremble before her father’s anger, she opposed him with courage and at last made him change his mind. Mary Cassatt gave up her social position and all thoughts of a thousand and a family, which in those times was unthinkable for a young lady. In the end, after long years of hard work and perseverance, she became America’s most important woman artist and the internationally recognized leading woman painter of the time.

What made Mary Cassatt’ s "struggle" to become a recognized artist especially hard（）

A．She was a woman.

B．Her father opposed her.

C．She had no social position.

D．She didn’t come from an artist’s family.
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